参数资料
型号: LTC3808EDE#PBF
厂商: Linear Technology
文件页数: 18/28页
文件大小: 0K
描述: IC REG CTRLR BUCK PWM CM 14-DFN
标准包装: 91
PWM 型: 电流模式
输出数: 1
频率 - 最大: 825kHz
占空比: 100%
电源电压: 2.75 V ~ 9.8 V
降压:
升压:
回扫:
反相:
倍增器:
除法器:
Cuk:
隔离:
工作温度: -40°C ~ 85°C
封装/外壳: 14-WFDFN 裸露焊盘
包装: 管件
LTC3808
APPLICATIO S I FOR ATIO
V X
V OUT
V X
V OUT
TIME
TIME
3808 F05b,c
(5b) Coincident Tracking
(5c) Ratiometric Tracking
Figure 5b and 5c. Two Different Modes of Output Voltage Tracking
For coincident tracking (V OUT = V X during start-up),
R TA = R A , R TB = R B
V X should always be greater than V OUT when using the
tracking function of TRACK/SS pin.
The internal current source (1 μ A), which is for external
soft-start, will cause a tracking error at V OUT . For example,
if a 59k resistor is chosen for R TA , the R TA current will be
about 10 μ A (600mV/59k). In this case, the 1 μ A internal
current source will cause about 10% (1 μ A/10 μ A ? 100%)
tracking error, which is about 60mV (600mV ? 10%)
referred to V FB . This is acceptable for most applications. If
a better tracking accuracy is required, the value of R TA
should be reduced.
Table 1 summarizes the different states in which the
TRACK/SS can be used.
Table 1. The States of the TRACK/SS Pin
TRACK/SS Pin FREQUENCY
is an edge sensitive digital type that provides zero degrees
phase shift between the external and internal oscillators.
This type of phase detector does not exhibit false lock to
harmonics of the external clock.
The output of the phase detector is a pair of complemen-
tary current sources that charge or discharge the external
filter network connected to the PLLLPF pin. The relation-
ship between the voltage on the PLLLPF pin and operating
frequency, when there is a clock signal applied to SYNC/
MODE, is shown in Figure 6 and specified in the electrical
characteristics table. Note that the LTC3808 can only be
synchronized to an external clock whose frequency is
within range of the LTC3808’s internal VCO, which is
1200
1000
800
Capacitor C SS
V IN
Resistor Divider
External Soft-Start
Internal Soft-Start
V OUT Tracking an External Voltage V X
600
400
200
Phase-Locked Loop and Frequency Synchronization
0
0.2
0.7 1.2 1.7
2.2
The LTC3808 has a phase-locked loop (PLL) comprised of
an internal voltage-controlled oscillator (VCO) and a phase
detector. This allows the turn-on of the external P-channel
MOSFET to be locked to the rising edge of an external clock
signal applied to the SYNC/MODE pin. The phase detector
PLLLPF PIN VOLTAGE (V)
3808 F06
Figure 6. Relationship Between Oscillator Frequency
and Voltage at the PLLLPF Pin When Synchronizing to
an External Clock
3808f
18
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